In the interest of editorial simplicity, this specification will refer to "bathroom tissue". However, where the context allows, it should be understood that the relevant technological principles apply also to the manufacture of all lightweight absorbent grades of paper, including, for instance, facial tissue and paper toweling.
Paper is generally manufactured by suspending cellulosic fibers of appropriate length in an aqueous medium and then removing most of the water. The paper derives some of its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most by far of the paper's strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this interfiber bonding, while necessary to the utility of the product, results in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drum thermal drying means with an adhesive/release agent combination and then scraping the web off of the Yankee by means of a doctor blade. Creping, by breaking a significant number of interfiber bonds, increases the perceived softness of resulting bathroom tissue product.
However, creping alone may not be sufficient to impart the optimum degree of softness to the bathroom tissue. Therefore, as reported for instance by Graef et alia in U.S. Pat. No. 5,225,047, by Van Phan in U.S. Pat. No. 5,217,576, and by Soerens et alia in U.S. Pat. No. 4,795,530, compounds such as quaternary amines that function as debonding agents are often incorporated into the paper web. As Soerens points out, cationic quaternary amines can be added to the initial fibrous slurry from which the paper web is subsequently made. Soerens teaches that it is preferable, however, to spray the chemical debonding agent onto the cellulosic web, after it is formed but before it is dried. Soerens indicates that a problem with cationic quaternary amines is that they substantially interfere with the adhesive/release agent combination normally employed to obtain proper adhesion of the cellulosic web precursor to the Yankee drum thermal drying means. Soerens teaches that this problem may be overcome by spraying the amines onto the partially dewatered web in such a way that the amines penetrate no more than 40% of the way through the thickness of the web. Soerens teaches that the presence of some sort of vacuum device at the point at which the amine is sprayed onto the web is essential to achieve and control the proper penetration of the debonding agent. The teachings of the Soerens patent indicate that in order to obtain soft strong bathroom tissue, the amines must not be sprayed onto the web at that stage of the papermaking process when the web is adhered to the Yankee.
Soerens refers to two earlier patents that deal with the addition of quarternary ammonium compounds to wet cellulosic webs: U.S. Pat. No. 2,756,647 and Canadian Patent No. 1,159,694. Like Soerens, the Canadian patent is concerned with the manufacture of creped bathroom tissue; this patent teaches spraying the partially dewatered web with an acidified debonding agent before adhering it to the Yankee dryer. U.S. Pat. No. 2,756,647 does not appear to be concerned with the manufacture of creped bathroom tissue. Although this patent refers to "soft paper tissue", there is no mention of creping in the patent. It is believed that this patent, which is assigned to Personal Products Corporation, is concerned with the preparation of coverstock for catamenial fluid absorbtion products. The patent teaches that aromatic quarternary ammonium bactericides may be sprayed onto paper backed by (but apparently not adhered to) what appears to be a small drum dryer. There does not appear to be any adhesive/release agent involved, as there would be in a creping context. Because there is substantial opportunity for undesirable interaction between softener compounds and release agents in the papermaking process, it is believed that the teachings of U.S. Pat. No. 2,756,647 are largely irrelevant to the present invention which does relate to creping with its concomitant adhesives and release agents.
U.S. Pat. No. 4,447,294 to Osborn discloses another way to avoid problems on the Yankee stemming from the use of quaternaries. Osborn applies the quaternaries to the product after it is creped from the Yankee.
The Soerens patent alleges that, in order to obtain the result indicated by the patent to be its objective, careful control of the amount of debonder solution applied--as well as of the vacuum applied to the wet web after the debonder solution is sprayed on it--is required.
It is an object of the present invention to provide a method for the manufacture of soft strong bathroom tissue that is simpler and more flexible that the method taught in the Soerens patent.
Spraying the web before adhering it to the Yankee dryer can result in the presence of debonder in the "white water" that is generally recirculated as carrier into the papermaking process. The debonder, which is cationic, is thought to interact with "trash" in the water, such as "fines"--that is, tiny bits of broken cellulose fibers--and oily or gummy substances--derived from the pulp--to form deposits that reduce tensile strength in the paper being produced and increase problems with runnability of the process.
It is also an object of this invention, therefore, to provide a method for the manufacture of bathroom tissue that avoids the problems that sometimes occur when softener is recirculated directly into the papermaking process.
One-ply bathroom tissue generally suffers from the problem of "sidedness"--that is, one side of the sheet is generally appreciably less soft than the other side. Sidedness is introduced into the sheet during the manufacturing process. In some cases, the sidedness is introduced intentionally, by means of a two-slice headbox. In such cases, one slice provides softwood fibers for strength and the other slice provides hardwood fibers for softness. The resulting sheet is stratified, with the softwood side being dramatically rougher than the hardwood side. Such paper is suitable for use in making two-ply products, in which the two rougher sides face each other and the two softer sides face outwardly. A more sophisticated procedure, described for example in U.S. Pat. No. 5,178,729 (Janda--incorporated herein by reference) involves the use of a three-slice headbox, in which the middle slice provides a layer with strength and, if desired, bulk characteristics, and the two outer slices provide soft hardwood layers. Even with three-layered laminated sheets, however, sidedness can be a problem, being imparted to the sheet during the drying procedure. The side of the sheet that was adhered to the Yankee and creped off is generally softer than the "air" side of the sheet. This two-sidedness is seen both in sheets that have been pressed to remove water and in unpressed sheets that have been subjected to vacuum and hot air (through-drying) prior to being adhered to the crepe dryer.
It is a further object of this invention to provide a method for the manufacture of one-ply bathroom tissue that shows a relatively low degree of "sidedness".